Rising temperatures mean that fish get warmer, and warmer fish need more oxygen to keep their bodies going. But warming waters hold less oxygen, so fish need to be smaller to compensate.

“Our study shows that climate change can lead to a substantial decrease in the maximum body weight of fish,” says Cheung to the BBC.

The research doesn’t mean that every fish you see swimming in the great big sea will be three-quarters of what it was before. Rather, the scientists looked at how the total average fish size will change with time. They considered not only the size of individual fish but changes in where various types of fish live and to the size of their populations. More little minnows moving into a region dominated by tuna would drop the average body size, for instance.

But, when looking at individual fish species, they found here, too, a decrease in fish sizes. Looking at one fish species at a time, they found that,

“…most (>75%) of the studied populations are expected to experience a reduction of their of 5–39%, with a median of 10% in all ocean basins. As a result of the higher rates of warming and reduction in oxygen content, the magnitude of decrease in individual is larger for fishes in the Pacific and Southern oceans, followed by those in the Atlantic, Indian and Arctic oceans.”

The new research takes a broad look at the possible changes to fish sizes given a steadily warming world, supplementing previouslaboratory and in-the-field research that identified similar effects of temperature on fish size. The scientists worry that these climate change-driven effects will stack alongside other issues, like over-fishing, to cause serious problems for global fish populations.

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About Colin Schultz

Colin Schultz is a freelance science writer and editor based in Toronto, Canada. He blogs for Smart News and contributes to the American Geophysical Union. He has a B.Sc. in physical science and philosophy, and a M.A. in journalism.

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